1. Field of the Invention
The invention relates to martensitic stainless steel of high strength which is applied to fields requiring rusting resistance and more particularly for use, for example, as a screw of superior screwing ability; a nail of superior driving ability and also rusting resistance; a cutter of superior rusting resistance and a .spring of superior rusting resistance.
2. Description of the Prior Art
Heretofore, a carbon steel special screw called a self drilling tapping screw 1 as shown in FIG. 1 has been used for a fixing process by screws on carbon steel products and surface treated steel sheets. And, for the purpose of the improvement of work efficiency and cost reduction, a direct fixing method has been put into practical use in which the screwing process is performed directly from the surface of a steel plate 2 without forming holes beforehand as shown in FIG. 2.
That is, this method applies a screw formed in the shape of a drill (a cutting edge) at the pointed end of it so as to fix the steel plate 2 to a lower steel construction 3 by simultaneously drilling and tapping them with the screw part.
Recently, however, as environmental conditions have become worse due to acid rain, etc., it is more strongly required to change from a carbon steel self drilling-tapping screw to one with high rusting resistance, i.e., a stainless one.
And recently, from the susceptibility and rusting resistance point of view, the area of application of stainless steel products is expanding widely into architecture or architectural material, vehicles, etc. In these cases, stainless steel products of this kind have been used in surface construction by means of spot welding or a fixing process by screws. However, changing of a stainless steel screw which is used in such a screwing process to a self drilling tapping screw could not be performed because of insufficient hardness. For this reason, heretofore, a fixing technique as shown in FIG. 3 has been applied unavoidably, where a steel construction 3 in which an under-hole 7 is previously provided for insertion of a screw and a stainless steel product 5 in which a middle-hole 6 is previously provided for passing of a screw are positioned so that both holes may be aligned and then fixed with a stainless screw 4 through the holes. However, for the purpose of the improvement of work efficiency and cost reduction, it has been more often required to change such a stainless screw to a self drilling-tapping screw. In this case, to screw into a steel plate of 5 mm or more in thickness, a cutting edge of the screw should be 500 or more in Vickers hardness and 400 or more for threads and roots of the screw.
Rusting resistance equivalent to SUS304 is also particularly required for a head of the screw because it is exposed on the surface of a steel sheet.
And, high toughness of 60 J/cm.sup.2 or more in impact value is required for both the head part and a shaft part of the screw in order to prevent them from being damaged when screwing.
Furthermore, a raw material for the screw is required so that work for forming the cutting edge, work for screw thread cutting and work for forming the screw head can be easily carried out.
As described above, a raw material must have characteristic such as high cold workability in working time, and such as high strength of 500 or more in Vickers hardness, rusting resistance equivalent to SUS304 and high toughness in a use.
Heretofore, attempts to use austenitic stainless steel having high workability and hardening property as such a material have been tried, however it is inferior in cold workability and the lifetime of tools.
Products made of austenitic stainless steel such as SUS305, SUSXM7, etc., which have been hardened by nitriding after cold working, are also on the market. However, such a surface treated material by nitriding is inferior in rusting resistance to SUS304.
Products made of martensitic stainless steel, SUS410, which has been treated with nitriding quench process after cold working, have also been suggested. However, they are inferior in rusting resistance to SUS304.
Furthermore, heretofore, martensitic stainless steel having a high quenching ability and containing no .delta.-ferrite, and which contains 0.15% of C; 0.2% of Si; 0.68% of Mn; 6.2% of Ni; 11.3% of Cr; 2.1% of Mo; 0.15% of N; 0.15% of Zr, has been suggested as a material having high strength, high toughness and high rusting resistance. However, the target characteristics have not been obtained because not only is it impossible to carry out cold working having a high reduction such as a heading process, etc., owing to lowering of Ac.sub.1 temperature (i.e., 560.degree. C.) causing increased softening resistance when annealing, but also screwing ability is inferior owing to lowering of quenched hardness of 500 or less (480) in Hv.
As described above, a material having all the characteristics mentioned above at the same time has not been found. Therefore, such a self drilling-tapping screw that is obtained by joining a tool carbon steel shaped like a drill to the tip of a screw prepared by hardening stainless steel such as SUS305 or SUSXM7 with cold working, is inevitably used.
Such a self drilling-tapping screw that is obtained by putting a plastic cap on the screw head of a carbon steel self drilling-tapping screw to give rusting resistance only to the screw head, is also used.
However, it could not be said that these techniques have achieved the desired goal in spite of the fact that development of such a screw as a single body is still proceeding, because they still cost too much.